1. Field of the Invention
This invention relates to a high efficiency speech coding apparatus, and a routing switching system employing such a high efficiency speech coding apparatus, and, more particularly, to a high efficiency speech coding apparatus which codes and decodes a digital speech signal with high efficiency between a digital exchange and a digital trunk circuit which have different transmission bit rates from each other and a transit switching system which includes such high efficiency speech coding apparatuses between each of a plurality of digital trunk circuits and a digital exchange.
2. Description of the Related Art
In transmission of high efficiency coded speech information which makes use of high speed digital private line or like services, where inter-office transit switching of a digital trunk circuit is performed by a digital exchange which also performs switching with an intra-office subscriber line, the bit rate of a signal transmitted in the trunk circuit (for example, a high efficiency coded speech signal of 16 Kbps) must be matched with the bit rate of a pulse coded modulation (PCM) speech signal of, for example, 64 Kbps.
Therefore, a high efficiency speech coding apparatus, including a coding section for coding a PCM speech signal of 64 Kbps into a high efficiency coded speech signal of 16 Kbps by high efficiency coding, and a decoding section for conversely decoding a high efficiency coded speech signal of 16 Kbps into a PCM speech signal of 64 Kbps, is conventionally provided at a junction between an exchange and a trunk circuit. Thus, upon transmission from the digital exchange to the digital trunk circuit, a signal is passed through the coding section, but on the contrary upon transmission from the digital trunk circuit to the digital exchange, a signal is passed through the decoding section.
The transit switching system, however, has a problem in that, since a speech signal is repetitively processed by high efficiency speech coding and decoding processes each time it is transit switched by an exchange, the speech quality of speech information is degraded and the encode and decode processing delay is increased as much.
A countermeasure to solve the problem is proposed, for example, in Japanese Patent Laid-Open Application No. Showa 63-185264 wherein, using an exchange for exclusive use having a function of controlling a high efficiency speech coding apparatus, when inter-office transit switching is to be performed, a control signal different from a main signal for speech is sent out from the exchange to the high efficiency speech coding apparatus, in which it passes through a rate conversion circuit or the like, bypassing the coding section and the decoding section to prevent repetitions of high efficiency speech coding and decoding.
With the system just described, however, since a control signal different from a main signal must be transmitted, for each transit switching, from an exchange which performs intra-office and inter-office transit switching, to a high efficiency speech coding apparatus, a special exchange which can send out such a control signal is required. Consequently, the system is disadvantageous in that a high cost is required for the exchange and, accordingly, a high cost is required for the entire transit switching system.